In order to meet the demand for ever-increasing data transmission rates, while ensuring that various kinds of equipment, such as networked computing equipment, can communicate with one another, new standards for high speed data transmission are continually being developed. For example, data networks operating in accordance with the fiber-distributed-data-interface (FDDI) standard, fiber channel, Asynchronous Transfer Mode (ATM), Synchronous Optical Network (SONET), Synchronous Digital Hierarchy (SDH), High Speed Ethernet or Higher Speed Token Ring Standard and their derivatives are capable of transmitting data at line rates of 16 Mb/s up to the Gigabit/s region via optical fiber.
However, the ongoing evolution of high speed data transmission systems poses a dilemma for owners of older equipment. Many distributed data systems utilize electric current carrying media, e.g., copper wire, for data communication. Such conductive media include coaxial cable, shielded-twisted pair cable (STP), or unshielded twisted pair cable (UTP). In metallic conductor-based networks, data transmission rates on the order of 16 Mb/s are known. To obtain data transmission rates on the order of those available via optical fiber communication, it has heretofore been necessary to replace the metallic conductors with optical fibers. Upgrading a metallic conductor-based network to an optical fiber-based network can be prohibitively expensive because of the cost of replacing cables and connectors.
In view of the foregoing, it would be desirable to have a device by which computing equipment could transmit and receive data at the high data rate provided by FDDI over traditional, metallic conductive media in order to avoid the expense and effort required to replace such media with optical fibers. However, such a goal is not readily achieved.
In order to transmit and receive data via a conductive medium, there are several constraints which must be satisfied. These constraints include maintaining electromagnetic emissions from the metallic conductors and associated components within permissible limits, minimizing the influence of interfering electromagnetic signals upon the data signals on the metallic conductors, compensating for distortion of the received signals that results from the transmission line characteristics of the conductive medium, and regenerating useful logic signals from the received signals.
Since metallic conductive media radiate electromagnetic energy, it is desirable to minimize the radiated energy in order to reduce signal loss and to reduce electromagnetic emissions. Emission reduction is of particular importance since regulatory authorities, such as the United States Federal Communication Commission, prescribe limitations upon the amount of power which may be radiated from computing equipment within particular frequency bands in order to prevent interference with other electronic devices. Methods have been developed to reduce the bandwidth required for transmitting digital signals at a given bit rate, to distribute the radiated power across a wide region of the spectrum so that the average power in any one region is below the prescribed limit, and to confine electromagnetic fields to the conductive medium.
Line coding techniques are used to reduce the bandwidth required to transmit serial data. The serial data may contain undesirable spectral peaks because of the presence of embedded timing signals or predefined bit sequences that are used for such functions as flow control and/or providing status information. In order to eliminate these undesirable peaks, a scrambler is used to randomize the serial data signal. The scrambler operates according to a deterministic scrambling algorithm so that the serial signal can be easily recovered from the scrambled signal. The resulting scrambled signal may then be converted to a multi-level format, such as a pseudoternary format wherein the polarity of successive low-to-high logic level transitions is reversed in order to lower the fundamental Fourier component of the scrambled signal.
The simplest approach to confining electromagnetic emissions is to use a shielded cable wherein a grounded metallic shield surrounds the conductors. Such shielded cable is often provided in the form of co-axial cable or as shielded twisted pair (STP) cable. The grounded metallic shield introduces a parasitic, distributed capacitance which attenuates signals transmitted over the conductors. However, shielded cable is less flexible and more expensive than unshielded twisted pair (UTP) cable.
In order to reduce electromagnetic emissions from UTP cable, a balanced, or differential, signal format is commonly used. A differential signal is one in which a voltage transition applied to one of the conductors is accompanied by a complementary transition applied to the other conductor. Ordinarily, the voltage transitions on each conductor are singly, or independently, compared to a reference level, such as ground at the receiving end of the cable. The reference voltage level, or baseline, can fluctuate as a result of a prolonged signal sequence having a non-zero average value. Additionally, the received differential signal often includes a common mode voltage signal which may be induced in the conductors by another source of interference along the cable route.
All electrically conductive media cause various types of distortion in a transmitted signal. Two common types of such distortion are amplitude dispersion and phase dispersion. Amplitude dispersion is the attenuation of the amplitude of the transmitted signal at a loss factor that varies with signal frequency and cable length. Phase dispersion is the propagation of signals at a speed which also varies with signal frequency and cable length. Since a digital signal is composed of a large number of frequency components, both of these dispersive effects limit the distance over which and the data transmission rate at which a cable may effectively transmit data.
In order to receive useful data from a signal transmitted over a metallic conductive medium, it is often necessary to equalize the received signal, or to compensate for the signal distortion caused by the transmission characteristics of the cable. The known equalizing circuits employ an amplifying circuit having gain and phase characteristics that vary in inverse proportion to the gain and phase characteristics of the transmission medium. The type of equalization that has been heretofore proposed for high-speed digital communication over a metallic conductor is an adaptive equalization system wherein the equalizing characteristic is automatically and continuously adjusted via a feedback loop that monitors a particular quality of the received signal, such as the average peak-to-peak voltage. The monitored average peak-to-peak voltage is also used as a reference level against which a received pseudoternary signal is compared in order to regenerate the original binary signal.
Adaptive equalization and conversion circuits possess several disadvantages in the context of highspeed conductive digital receivers. One disadvantage is that the feedback loop necessarily introduces a finite delay, or adaptation time, into the equalization process. In the reception of broadband signals, adaptation delay can cause significant distortion of the signal rather than the desired correction of transmission distortion. Another disadvantage of the known adaptive equalizing circuits is that they are susceptible to errors that result from the presence of common mode voltages on the differential input lines. Since the conversion from differential pseudoternary to binary is effected by comparing amplified versions of the individual differential signals with a time-averaged peak-to-peak value of the received signal, common mode components of the received signal are amplified and compared to an average signal value that is no longer relevant to the instantaneous signal. Crosstalk from adjacent signal transmission lines or other sources of noise can cause false adaptation in equalizers that are sensitive to variations in peak-to-peak voltage.